1. Field of the Invention
The present invention relates to a method for setting a free-state diameter of a metal ring of a metal belt for a continuously variable transmission upon cutting of the metal ring at a predetermined value in order to apply a residual stress to increase the fatigue life of the metal ring.
2. Description of the Related Art
A metal belt used in a continuously variable transmission typically includes a plurality of metal elements mounted on metal ring assemblies, each metal ring assembly having a plurality of metal rings laminated on one another. Accordingly, a driving force is transmitted by a pushing force between the metal elements, which are in contact with one another, in a state in which the metal belt has been reeved around a driving pulley and a driven pulley. A stress applied to each of the metal rings supporting the metal elements in the metal belt is varied periodically during traveling of the metal belt along the driving pulley and the driven pulley. Moreover, the applied stress is different between an inner peripheral surface and an outer peripheral surface of the metal ring. If the stress applied to the inner peripheral surface of the metal ring and the stress applied to the outer peripheral surface are not uniform, a face of the metal ring which receives a large stress fatigues early, which causes a reduction in the overall fatigue life of the overall metal ring assembly.
Conventional metal belts are disclosed in Japanese Patent Application Laid-open No. 63-20945 and Japanese Patent Publication No. 7-110390, wherein a residual stress is pre-applied to each metal ring so the stresses applied to the inner and outer peripheral surfaces of the metal ring are as uniform as possible. Thus, any stress generated by operation of the metal belt is countervailed by the residual stress and the fatigue life of the metal ring is prolonged.
The largest stress on the metal ring during operation of the continuously variable transmission varies in accordance with the free-state diameter of the metal ring. The term xe2x80x9cfree-state diameter of the metal ringxe2x80x9d is defined as a diameter of the metal ring measured in a free state of the metal ring in which a sum total of moments generated by the residual stress upon cutting of the metal ring is zero. The metal ring is manufactured such that when the free-state diameter is increased, the largest stress on the outer peripheral surface of the metal ring increases, while the largest stress on the inner peripheral surface decreases. In the above-described conventional metal belt, a peak value of the largest stress on the metal ring is suppressed to a lower value to prolong the fatigue life by setting the free-state diameter such that the largest stresses on the outer and inner peripheral surfaces are equal to each other.
In the above-described conventional metal ring, however, the largest stress on the metal ring is calculated based on a bending stress at a portion where the metal belt is wound on the pulley, and a stress based on the tension of the metal belt is not taken into consideration. Accordingly, the correlation between the free-state diameter and the fatigue life of the metal ring is unsatisfactory.
It is an object of the present invention to overcome the above-described drawbacks of the conventional metal belts.
It is also an object of the present invention to set a free-state diameter enough to maximize the fatigue life of each metal ring of the continuously variable transmission.
To achieve the above objects, there is provided a method for setting a free-state diameter of each metal ring of a metal belt of a continuously variable transmission, upon cutting of the metal ring at a predetermined value that depends on a residual stress for increasing a fatigue life of the metal ring in order to apply the residual stress to the metal ring. The method includes the step of calculating variations in the stresses on the inner and outer peripheral surfaces of the metal ring during operation of the continuously variable transmission. An amplitude and a middle value of the stress on the outer peripheral surface of the metal ring is then calculated. A corrected amplitude of the stress on the outer peripheral surface from the amplitude and the middle value of the stress on the outer peripheral surface is then calculated. An amplitude and a middle value of the stress on the inner peripheral surface of the metal ring is calculated. A corrected amplitude of the stress on the inner peripheral surface from the amplitude and the middle value of the stress on the inner peripheral surface is calculated. A free-state diameter is set based on the corrected amplitude of the stress on the outer peripheral surface and the corrected amplitude of the stress on the inner peripheral surface.
With the above configuration, the free-state diameter, upon cutting of the metal ring, is set based on the corrected amplitude of the stress on the outer peripheral surface, which is calculated from the amplitude and the middle value of the stress on the outer peripheral surface of the metal ring and the corrected amplitude of the stress on the inner peripheral surface, which is calculated from the amplitude and the middle value of the stress on the inner peripheral surface of the metal ring. Therefore, an appropriate free-state diameter can be set in consideration of a stress based on a tensile load of the metal ring and a stress based on a bending load of the metal ring, wherein the life of the metal ring is effectively prolonged.
Alternatively, the free-state diameter can be set so the corrected amplitude of the stress on the outer peripheral surface of the metal ring and the corrected amplitude of the stress on the inner peripheral surface are equal to each other.
With the above configuration, the free-state diameter is set so the corrected amplitudes of the stresses on the outer and inner peripheral surfaces of the metal ring are equal to each other. As a result, the fatigue lives of the outer and inner peripheral surfaces are uniform, wherein the life of the metal ring is prolonged even further.
The free-state diameter may also be set based on the corrected amplitude of the stress on the outer peripheral surface of the metal ring and the corrected amplitude of the stress on the inner peripheral surface in a TOP-ratio state of the continuously variable transmission.
With the above configuration, the free-state diameter is set based on the corrected amplitudes of the stresses on the outer and inner peripheral surfaces of the metal ring in the TOP-ratio state of the continuously variable transmission. Therefore, it is possible to prolong the life of the metal ring at the time when the metal ring is in a most severe operational state.
A region between the free-state diameter at which the corrected amplitudes of the stresses on the outer and inner peripheral surfaces of the metal ring in the TOP-ratio state of the continuously variable transmission are equal to each other and the free-state diameter at which the corrected amplitudes of the stresses on the outer and inner peripheral surfaces of the metal ring in an OD-ratio state of the continuously variable transmission are equal to each other is established as a region where the free-state diameter is appropriate.
With the above configuration, the region between the free-state diameter at which the corrected amplitudes of the stresses on the outer and inner peripheral surfaces of the metal ring in the TOP-ratio state are equal to each other and the free-state diameter at which the corrected amplitudes of the stresses on the outer and inner peripheral surfaces of the metal ring in an OD-ratio state is established as the region where the frees-state diameter is appropriate. Therefore, the life of the metal ring is effectively prolonged in the region from the TOP state in which the load of the metal ring is largest to the OD state in which the service frequency is highest.
Alternatively, in a region between the free-state diameter at which the corrected amplitudes of the stresses on the outer and inner peripheral surfaces of the metal ring in a TOP-ratio state of the continuously variable transmission are equal to each other and the free-state diameter at which the corrected amplitude of the stress on the inner peripheral surface of the metal ring equal to the corrected amplitude of the stress on the outer peripheral surface of the metal ring in a state in which the ratio of the continuously variable transmission at the above-described free-state diameter assumes a value in the OD state is generated in the TOP-ratio state of the continuously variable transmission is established as the region where the free-state diameter is appropriate.
With the above configuration, the region between the free-state diameter at which the corrected amplitudes of the stresses on the outer and inner peripheral surfaces of the metal ring in the TOP-ratio state are equal to each other and the free-state diameter at which the corrected amplitude of the stress on the inner peripheral surface of the metal ring equal to the corrected amplitude of the stress on the outer peripheral surface of the metal ring in a state in which the ratio of the continuously variable transmission at the above-described free-state diameter corresponds to the OD state is generated in the TOP-ratio state, is established as the region where the free-state diameter is appropriate. Therefore, the upper limit of the corrected amplitude of the stress can be suppressed to a predetermined value of less in the region from the TOP state in which the load of the metal ring is largest and the OD state in which the service frequency is highest, wherein the life of the metal ring is effectively prolonged.
The above and other objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiment taken in conjunction with the accompanying drawings.